The present invention relates to an optical sheet that has a function of improving a directivity of light raysxe2x80x94that is, an optical functionality sheet composed of a microlens array and a reflective film or light-blocking filmxe2x80x94and a planar light source, and liquid crystal display apparatus using this sheet.
Recently, image display apparatuses, of which liquid crystal display panels are representative examples, have come into wide general use as display means for personal computers, workstations, and so forth. In terms of the quality of the images displayed using such display apparatuses, the characteristics demanded are high brightness and high display contrast, together with a wide angle of visibility.
In order to realize the above described image quality, an installation of a light-diffusing plate between a liquid crystal display panel and a back light source has been disclosed in JP-A-6-95099 specification. Also, a method has been disclosed for enlarging an angle of visibility by installing a sheet that has a microlens array and a light-blocking layer on a liquid crystal image display surface.
Moreover, in JP-A-10-39769 specification, a screen that coordinates array patterns of microlenses and light-blocking film has been disclosed as the screen of a rear-projection type projection apparatus.
When the above described wide-angle-of-visibility sheet is used in a liquid crystal display apparatus, characteristics required of the light source are high directivity and nearly collimated light rays (parallel light rays). The reason for this is that, if the light rays were not parallel, the rays would not be converged sufficiently by the microlenses, and would be projected onto areas where the light should be blocked. This would cause light loss and reduce the brightness of the image display apparatus.
General planar light sources (backlights) for the above described use employ various kinds of diffusion plates that diffuse light beams randomly, in order to achieve uniformity of brightness of the light emitting surface, and the beams emitted from this light emitting surface do not have directivity.
Furthermore, a so-called louver sheet, comprising many rows of light-blocking walls aligned with the direction of travel of the beams, is known as a light source that has directivity. By means of a louver sheet, it is possible to obtain a planar light source that has arbitrary directivity, by taking emitted light with a spread of 120 degrees or more in an emitting surface, for example, and cutting off beams traveling in other than the required direction by means of light-blocking walls. However, this sheet has low light usage efficiency, and is not suitable for the image display apparatus which is an objective of the present invention.
As another method, a method has been disclosed whereby a prism sheet arrayed with a large number of minute triangular prisms is placed on the surface of a photoconductive plate. This is achieved by controlling to some extent the direction of emission of the beams. This prism sheet enables directivity of the order of xc2x130 degrees to be obtained, but does not meet the requirements of the image display apparatus which is an objective of the present invention.
A light ray directivity sheet that eliminates the above described problems, has high light directivity and light usage efficiency, is moreover of thin shape, and enables a uniform planar light source to be obtained, and a directional planar light source using this, have been disclosed in JP-A-9-1675133 specification and JP-A-10-241434 specification.
This method consists of a light ray directivity sheet, one surface of which comprises a group of microlenses in which unit lenses are arrayed, and on the other surface of which a light ray blocking film (reflective film) is formed, wherein, at least, areas in the vicinity of the focal points of light rays entering from the microlens group side of the above described light ray blocking film are made apertures. By positioning the surface of this light ray directivity sheet on which the light ray blocking film is formed on the light source side, and positioning the microlens surface on the viewing side (liquid crystal display element side), a planar light source is obtained that is given directivity by the operation of the microlenses.
By using a planar light source fitted with the above described light ray directivity sheet using microlenses, and an angle of visibility enlargement sheet using similar microlenses, it is possible to realize an image display apparatus with high brightness and a wide field of view.
However, with any sheet, there are many problems that need to be solved in the actual construction process.
First of all, there are major limitations in the construction of an angle of visibility enlargement sheet. That is to say, when a microlens array and a light-blocking layer (black matrix) for suppressing the re-reflection of external light reflected by the surface of this microlens array are combined, it is essential for the layout patterns of the microlenses and light-blocking layer to be precisely positioned relative to each other, since a slight misalignment will halve their function.
Known common methods for forming a light-blocking layer include forming as a thin metal film, and a method whereby a photosensitive resin film, in which a pigment such as carbon black has been dispersed or in which a black or other dye has been dissolved, is formed on a substrate, and is patterned by means of photolithography
However, if the light-blocking layer and the microlenses are formed by totally independent processes, and the two are combined later, it can be said that it is difficult to align the two accurately within several xcexcm. When the size of the microlenses is very small (several tens of xcexcm), in particular, accurate alignment is extremely difficult.
On the other hand, as a solution to the problems relating to an angle of visibility enlargement sheet, sensitization of a photosensitive layer by energy ray irradiation via optical elements (for example, a microlens array) corresponding to a black matrix pattern, and forming a black matrix of the desired pattern, has been disclosed in JP-A-10-246804 specification.
As the microlenses and the transparent parts of the light-blocking layer are formed by means of self-alignment, an advantage of this method is that it is easy to coordinate the respective pattern positions precisely, but the following problem arises in realizing this.
Namely, with the above described method, from the viewpoint of the work processes, a positive-type resist is generally used whereby the parts irradiated with energy rays are sensitized and become soluble in a solvent. However, in order for the light-blocking layer to be formed simultaneously by this method, the use of a non-transparent material containing carbon black, or a black dye or pigment, etc., in the above described resist is assumed. Therefore, the transmittivity of the energy rays, and especially the light rays used for pattern forming, is decreased, and it is difficult to obtain a prescribed pattern.
Therefore, the problem arises of it being necessary to spend a long time on energy ray irradiation, or to make the resist film thin, in order to compensate for the fact that the photo-transmittivity is low. Thus, the exposure process is time-consuming, and it is difficult to obtain a light-blocking film with a high optical density (high light-blocking capability).
Another method is one in which, after a layer constituting the light-blocking layer has been formed using a negative-type resist, the light-blocking layer is patterned by means of photolithography. However, the problem with this method is that the work processes are even more complex, making it impractical.
On the other hand, the same kind of problems as described above also arise with regard to a light ray directivity sheet using microlenses. That is to say, the array patterns of the microlenses and the light-blocking layer must be accurately aligned in order for the function to be fully implemented. This is because any misalignment reduces the parallelism (collimation) of the emitted beams, with a resulting problem of lower emission efficiency of the planar light source.
A reflective film, such as a metal film or titanium oxide, has been proposed as the light-blocking film of the above described light ray directivity sheet, and a self-alignment method, using lenses, has been proposed as the manufacturing method, as in the case of an angle of visibility enlargement sheet.
That is, a negative-type photo-resist is applied to the opposite surface from that with the group of microlenses, and the resist is exposed and developed by irradiation with parallel ultraviolet light from the lens formation surface side. By this means, a band-shaped pattern is formed on the parts corresponding to the lens focal points.
Next, a film that constitutes the light-blocking film is formed upon this. To be specific, a coating material in which titanium oxide is dispersed in acrylic resin is applied, or a film of metal, such as aluminum, is applied by vapor deposition, or a coating material in which carbon black is dispersed in acrylic resin is applied. Then, part of the area of the resist on which the band-shaped pattern has been formed (the projecting area) is cut away. The band-shaped pattern is completely removed with resist remover.
The above described method is called the lift-off method. This method makes it possible to obtain a light ray directivity sheet on which areas in the vicinity of the microlens focal point are made apertures, but it involves many processes, and also presents the following problems.
Namely, it is necessary to make the resist film thickxe2x80x94a problem characteristic of lift-offxe2x80x94and it is difficult to obtain a high-precision pattern with a photolithographic process. Also, an excessive load is applied to the resist in the process for forming the light-blocking film, and complete lift-off of the resist is difficult, among other things.
In order to solve the above described problems, and to obtain functionality sheets with excellent optical characteristicsxe2x80x94that is, a light ray directivity sheet and wide-angle-of-visibility sheetxe2x80x94the present invention employs a method whereby a patterned transparent conductive film is used, and a light-blocking film or reflective film is formed thereupon.
That is to say, in the present invention, a transparent conductive film is formed on the other side of a transparent member on which microlenses are formed. Next, a positive-type photo-resist film is formed on this transparent conductive film, and then so-called self-alignment exposure is carried out, whereby exposure is performed from the side on which the microlenses are formed, and aperture areas (transparent portions) are formed on the transparent conductive film by removing the resist, by development, from the lens convergence areas. After this, the transparent conductive film at the aperture areas is removed using an etching method. Lastly, with the transparent conductive film as an electrode, a light-blocking film is formed on this transparent conductive film using a method such as electrodeposition coating, metal plating, electrolysis, or electroforming.
Patterning can be performed easily and with high precision using means whereby patterning is performed by means of light irradiation of the photo-resist film from the side on which the microlenses are formedxe2x80x94that is, self-alignment exposure meansxe2x80x94via the above described transparent conductive film. Also, as electrical means such as electrodeposition is used for formation of the light-blocking film, with transparent film as an electrode, it is possible to form a colored film with low transparency, such as a black light-blocking film, a white diffuse reflection film, or a metal film, on a prescribed area with high precision, and to a degree of thickness as necessary.
Therefore, according to the present invention, it is possible to manufacture, with high productivity, an optical functionality sheet provided with microlenses and a diffusive white reflecting film or a light-blocking film, and also a directional planar light source and liquid crystal display apparatus using these.